The molybdo-enzymes xanthine oxidase and aldehyde oxidase occur in the liver; nitrate reductase and nitrogenase are important enzymes in the metabolism of inorganic nitrogen. Little is known about the structures of these large enzymes or about the role of molybdenum in their biochemical reactions. The objectives of this research are to investigate structural models for molybdo-enzymes. Particular attention will be given to the preparation of monomeric amino acid and polypeptide complexes of molybdenum. The structures of the complexes will be determined by x-ray crystallography and the physical and chemical properties of structurally characterized compounds will be related to those of the enzymes.